The present invention relates to improvements in and relating to a process for the preparation of spherical particles of halogen-containing ethylene resins, showing heavier values of apparent specific gravity.
Generally speaking, the halogen-containing ethylene resin shows lesser swelling ability as well as lesser solubility to the monomer, and thus, it is liable to provide rather porous resin particles of lesser specific value, when they have been prepared by the conventional suspension- or block polymerization.
With the resin particles thus prepared and of lesser specific gravity as above mentioned and as used for the supply material to melt-forming machines, not only the feed rate and thus, the extrusion rate will become correspondingly worse, but also the rate of voids formation of the shaped products therefrom by sintering will be subjected to a considerable fluctuation, thereby giving rise to the disadvantageous formation of air bubbles to a considerable degree and reducing the production efficiency to a substantial degree. As a further example when the resin is used as the material for the powder coating process, the coatings have only lesser affinity to the substrate bodies and the resulted coated layers are rather thin and frequently even pin-holed.
For converting these apparently lighter, halogen-containing resin particles into those having heavier specific gravity, it has already been proposed to process the former in such a way that these defective porous resin particles are dispersed in an aqueous phase and then heated in an autoclave in the presence of an inorganic suspension protector agent up to a higher temperature near the melting point of the resin (for instance, refer to Japanese Open Patent Specification Sho-51-150569).
This proposed conversion or improving process is rather superior from such a view point that the desired spherical and heavier resin particles can be provided at a rather high yield and lesser production cost, when comparing with still more conventional processes wherein resin monomer is after-polymerized with the porous resin particles during the polymerization step; the porous resin particles are baked by contact with a stream of high temperature gaseous medium; or they are melt-pelletized and then subjected to a freeze pulverization step.
However, we have experienced that the above proposed process represents such a defect that it must necessarily use a pressure vessel by virtue of the very use of water as dispersant. In addition, in this case, the autoclave or pressure vessel must be made of such higher grade as for avoiding excessive thermal distorsion as liably to be met during the resin particle-refining process. In addition to this defect, giving rise to a substantial shortening of durable life of the pressure vessel, rapid heating and cooling must preferably be avoided for extending the effective life thereof. Therefore, the vessel material must be selected carefully and specifically from high cost, high grade, high tension steels, thus representing a substantial defect. By virtue of the requisite use of water as dispersant for the formation of the aqueous phase to be employed and since the water represents a large heat capacity appearing each time for dispersion, suspension, heating, cooling and discharging of the resin material and thus requiring a long processing period and too many processing steps which mean a substantial processing defect, especially when considering the whole process from an economical point of view. This defect will be further accentuated by virtue of the batch processing mode of the process which must be necessarily be employed for the aimed purpose.